In the past there have been many attempts to provide packing materials for steam service. Because of the high pressure, high temperature superheated steam involved, ordinary packing materials cannot be utilized. Rather wire-reinforced asbestos-jacketed packings have been commonly utilized in steam valves. In automatic control steam valves, not only is steam integrity important due to 2000 psi and higher steam, but also valve stem erosion causes failure due to constant cycling, with the erosion being primarily due to wear produced by over use of reinforcing wire.
In general, prior steam service packings have been made on round braiders. Typically they use a resilient or pliable core material completely surrounded by a braided material in which the braid contains wire, usually Inconnel. Thus the packing is surrounded with reinforcing wire, which under pressure slices through the braid, contacts the valve stem and scores it because the Inconnel wire is harder than the commonly used steel or stainless steel valve stems. This causes leakage and subsequent catastrophic failure if the packing is not replaced frequently. Note the abraided particles from the valve stem also can cause packing failure, which with high pressure steam presents a very serious hazard, especially to those employed to repair a leak. As will be appreciated, when automatic steam control valves are used in electric power generation plants, oftentimes the turbines cannot be shut down, which necessitates repair in the presence of live steam presenting a hazardous leak stopping operation. It is thus necessary that the packing cause as little damage as possible to the valve stems while at the same time preventing steam leakage so that long packing life is assured.
Prior steam service packings include Crane style 187-I or Chesterton style 1500 which utilize a so-called "plastic" or flexible core of high purity asbestos, graphite flake, rubber, zinc and oil surrounded by wire-reinforced asbestos which is braided around the plastic core. The circumferential wire reinforcement of these packings and packing of similar construction present an overly abundant amount of wire to the valve stem, and with constant cycling of the valve stem requires repacking and valve reconstruction as early as three months due to valve stem wear.
Moreover, because these packings contain asbestos there is a problem when temperatures exceed 700.degree. F. in which the water of hydration in the asbestos is driven away. This results in the production of ovine powder. When this occurs the entire packing looses water and shrinks. Also the packing hardens which prevents adjustment of the packing by tightening the gland bolts. Moreover, there are removal problems due to the virtual disintegration of the packing with exposure to the high temperature steam.
Thus, aside from wire abrasion problems, at temperatures above 600-700.degree. F. in superheated steam the asbestos fibers begin to loose their water of hydration, forming ovine powder as has been noted in the EPRI valve system packing report by Stone & Webster Engineering Corporation, 1982. This results in shrinkage of the packing in the stuffing box, allowing leakage of steam, and requiring adjustment of the packing to stop the leakage. As noted in the report, the rubber and other binders in the packing harden, making adjustment difficult, if not impossible. As a result of the steam leakage, some of the asbestos fibers from the packing are carried into the air. Tests have found that some of these asbestos fibers carried into the air may be small enough to be respirable. These are the friable asbestos "fly", which has been named a carcinogen in 29 CFR 1910 and 1926 and for which specific standards have been set. As a result of these standards, and the hazards presented by handling asbestos packings, their use has been discouraged.
Regardless of the problems with asbestos, all these packings use an excessive amount of reinforcing wire, because all the yarn woven around the core is provided with wire. This results in excessive abrasion to the valve stems because all braiding in contact with the valve stem surface contains the reinforcing wire.
In an effort to solve all but the excessive wire problem, fiberglass reinforced with wire has been utilized, with the fiberglass provided in the core material to replace the asbestos and also as the woven material around the core for the same purpose.
The problem with fiberglass is that its relative long fibers break under changing pressures and temperatures and vibration, conditions which exist in the high temperature steam generating systems in which these packings are used. This again results in shrinkage. Moreover, the packing loses strength and suffers both the above removal problems and the utilization of an over amount of wire reinforcing because each overbraided yarn in these so called asbestos replacement packings contain reinforcing wire.
In order to solve the problem of packings using the pure fiberglass construction to replace asbestos, plastic cores with carbon fibers added to fiberglass, rubber, oil and zinc have been utilized for the core, and the over-braided structure contains some carbon fiber along with the wire and fiberglass.
Again the problem with such a packing is that it becomes extremely hard in service. Also shrinkage due to oil evaporation occurs and the oil also carbonizes which contributes to the hardening, a lack of adjustability and a shorter life span vis-a-vis the aforementioned asbestos braids based upon the experience of power generating plants. Here there are also removal problems and the problem of an overage of wire reinforcing material which causes abrasion on valve stems.
As another attempt to solve the problems with fiberglass, Chesterton style 1000 packing goes back to the use of asbestos, but uses a plaited or braided core of high purity asbestos instead of a plastic core. Again the core is provided with an outer braid of wire-reinforced asbestos. Here the plated core contains no oil or rubber which eliminates the hardening problems. However, an overage of wire is still in evidence. Moreover, there is shrinkage due to the drive off of the water hydration in the asbestos. The solution by Garlock style 1298 packing is the introduction of PBI (polybenzimidazole), in which PBI is substituted for asbestos and is braided about a carbon or graphite core. Additionally carbon fiber is utilized in the outer braid, along with wire and the PBI. The problems with this type of packing is that again too much wire is utilized. Shrinkage is also a problem which occurs with PBI above 700.degree. F.
By way of further background, non-wire reinforced packings are used for low pressure applications in which a sturdier secondary yarn is braided on diagonal tracks to give packings strength, especially at their edges. Such diagonally-reinforced packings are described in U.S. Pat. Application Ser. No. 943,950, filed Dec. 18, 1986 and now U.S. Pat. No. 4,802,398 assigned to the assignee hereof. Whether single or double diagonals are reinforced, none of the packings described in this Patent Application are used for steam service. Nor are they wire-reinforced.
By way of additional background as to braiding techniques, it should be noted that the Parker Company produces a packing style in which carriers on the diagonal tracks carry a differently-colored yarn. This braid differs from conventional braid in that only every other carrier on a given track is provided with the differently-colored yarn. The result of this braiding technique is mostly decorative so as to provide a braid which has alternating light and dark areas.
Were one to provide the Parker braid with wire-reinforced yarn on the carriers utilized in a 3 track braider for the differently colored yarn, one might produce a reinforced packing. However, there would be a foot print on the valve stem which would permit steam passage through the non-reinforced braided portions. In other words, were the Parker braid provided with wire-reinforced yarn for the carriers involved in producing their braid, the resulting braid might not be of any use in steam service because non-reinforced channels or voids might be left running down the valve stem, thus permitting massive steam leakage.